With an increase in capacity of information processing in recent years, various information recording technologies have been developed. In particular, the surface recording density of an HDD using magnetic recording technology is continuously increasing at an annual rate of approximately 60%. In recent years, an information recording capacity exceeding 320 gigabytes per platter has been desired for a magnetic recording medium with a 2.5-inch diameter for use in an HDD or the like. To fulfill such demands, an information recording density exceeding 500 gigabits per square inch is desired to be achieved.
Important factors for increasing recording density of the perpendicular magnetic disk include, for example, an improvement in TPI (Tracks per Inch) by narrowing the track width, ensuring electromagnetic conversion characteristics, such as a Signal-to-Noise Ratio (SNR) and an overwrite characteristic (OW characteristic) indicating ease of writing of a signal at the time of improving BPI (Bits per Inch), and further ensuring heat fluctuation resistance with recording bits decreased due to the above. Among these, an increase in SNR and an improvement in overwrite characteristic in a high recording density condition are important.
The granular magnetic layer is effective in increasing the SNR because an oxide phase and a metal phase are separated to form fine particles. For increasing recording density, the particles of the granular magnetic layer are required to be microfabricated. With microfabrication, however, magnetic energy is also decreased, thereby causing a problem of heat fluctuations. On the other hand, to avoid the problem of heat fluctuations, magnetic anisotropy of a medium is required to be increased. With an increase of magnetic anisotropy, a coercive force is also increased, thereby making it difficult to write. That is, while prevention of heat fluctuation and improvement of the overwrite characteristic are to be intrinsically traded off for each other, there is a demand for enhancing both of these.
Patent Document 1 suggests a write auxiliary layer (corresponding to an auxiliary recording layer) formed above or below a main recording layer and in contact with that main recording layer. In Patent Document 1, the main recording layer is a magnetic layer having a granular structure, and the write auxiliary layer is made of a CoCr alloy (for example, CoCrPtB). In Patent Document 1, by providing the write auxiliary layer, the overwrite characteristic and heat fluctuation resistance can both be improved.